It is shown that ohmic efficiency is limited to 50% for self-field quasi-steady plasma acceleration and for impulsive acceleration in small scale devices. This limit also applies when a constant bias field is added to the self-field but it can be removed by using a bias field that decreases in the acceleration direction. Tests were conducted with a bias field thruster in an attempt to demonstrate increased efficiency. It consisted of a V-shaped channel placed between pole pieces of a permanent magnet having the same divergence angle as the channel; electrodes were located in the upper and lower surfaces of the channel. Operation was quasi-steady for a current discharge period of 800 microseconds. Gas injection was continuous and was done in two ways - in one mode it was introduced through hollow electrodes, in the other it entered the channel through a narrow slit. In the final version tested the channel was made of alumina and tungsten electrodes were used. Impulse enhancement due to the bias field was determined by suspending the magnet from piano wires and measuring its deflection when the thruster was fired. For argon injection through the electrodes, the current was carried down-stream from the magnet in the usual self-field discharge mode and no magnet deflection occurred. For axial injection of argon a large magnet deflection occurred. This was found to be due to current carried on the sidewalls which was stabilized by the sidewalls and which produced severe ablation of the alumina. When a mixture of hydrogen and nitrogen was substituted for argon, no magnet deflection occurred.